The present invention relates to sealing the space between the internal floating roof and the inner tank wall of a storage tank for volatile or hazardous liquids. The present invention provides a means for protecting the various seal mechanisms utilized with internal floating roofs and extending the life span for those sealing mechanisms.
Bulk fluids, such as petroleum, other fuel products and by-products, and chemicals are often stored in large cylindrical tanks. These tanks are commonly designed with internal floating roofs or bulk fluid covers to minimize product losses to the atmosphere. A critical part of the internal floating roof is the annular or rim space between the perimeter of the internal floating roof and the inner wall of the storage tank. For an average floating roof, approximately one-third of the evaporation loss of the bulk fluid can be attributed to those vapors that escape past the rim seal.
Three types of rim seals are known in the industry. These types of seals have been tested for their efficiency in reducing evaporative losses from the rim space. The seal types are: liquid-mounted resilient-filled seal; mechanical shoe seal; and vapor-mounted resilient-filled seal. These seals have been used for decades and their relative length of service is known. Testing has revealed that the most efficient seal is a liquid-mounted seal, followed by a mechanical shoe seal and finally a vapor-mounted seal. Seal efficiency has been shown to increase when a secondary seal is placed above the first seal. The Environmental Protection Agency and most state environmental regulations require either a single liquid-mounted seal or a double seal where a secondary seal is placed above a mechanical shoe or vapor-mounted seal.
Experience has shown; over the years that the various seal types have different life spans. The best seal type, the liquid-mounted seal, has the shortest life span, usually less than one year. The mechanical shoe seal has the longest life span, often more than twenty years. The vapor-mounted seal has a life span lasting five years on average. The life span is directly related to the frictional effect of the inner tank wall on the outer seal material as it continually comes into contact with the metal wall.
An early description of the use of a mechanical shoe type seal coupled with a vapor barrier may be found in U.S. Pat. No. 5,372,270. This patent also describes the use of a secondary seal in the form of a wiper blade mounted above the primary vapor-mounted seal. Protecting the lower vapor-mounted seal is its mounting to a mechanical shoe at the tank wall. However, the wiper seal does not have any protection from the reversal of movement flexure or from the frictional contact with the metal tank wall.
Another description of the seal types used with internal floating roofs can be found in U.S. Pat. No. 6,164,479. This patent describes dual vapor-mounted barriers spanning the rim space between the outer wall of the floating roof and a mechanical shoe riding against the tank wall. While the invention described in the '479 patent is the horizontal mounting of the hanger-pushers, the use of the vapor-mounted seals in tandem is disclosed where the primary seal is mounted below the liquid level and the fabric is permitted to rise above the liquid level and the secondary seal is mounted entirely above the liquid level in the vapor space.
Both of these early sealing systems required dual or tandem seals to achieve the reduction in product evaporation to satisfy the industry and its regulators. It is an object of the present invention to provide a rim seal that combines the efficiency of a liquid-mounted seal with the longevity of a mechanical shoe seal, thus increasing the life span of the liquid-mounted seal far beyond its present annual replacement.
The problems with both sealing systems was that in order to inspect the primary seal one was required to remove the secondary seal exposing the vapor collection space to the atmosphere and potentially endangering the human inspector. It is, therefore, an object of the present invention to provide a seal that is quick and safe to inspect.
It is yet another object of the present invention to provide a rim seal that is simple to manufacture and readily simple to install onto floating roofs, including retrofitting existing steel or aluminum floating roofs to carry the rim seal of the present invention.
It is also an object of the present invention to provide a rim seal that is compatible for use with any type of product being stored. Other objects will appear hereinafter.